This invention relates to load substitution circuits and more particularly to an improved dual polarity circuit for energizing a reserve lamp in the event of a failure of a primary lamp located, for example, in a vehicle.
Incandescent lamps are commonly used for safety lighting on vehicles. For example, incandescent lamps are used as tail lights and brake lights on automobiles, trucks and trailers and incandescent lamps are used as marker lamps to outline the upper edges of semitrailers. When an incandescent is new, the filament, which is typically formed from tungsten, is ductile and is not affected by vibration. As the lamp is used, the filament becomes brittle and may break when subjected to vibration. In many vehicle applications, for example, lights used on trucks and semitrailers, tungsten filament incandescent lamps have a very limited lift expectancy due to vibrations caused by stiff vehicle suspension systems.
Laws require that semitrailers have marker lights spaced around the periphery of the trailer body. Laws often provide for stiff fines if a driver operates a semitrailer at night with burnt out marker lights. Consequently, it is often necessary for a truck driver to stop at night and call a service station to replace burnt out marker lights. This is extremely expensive both in paying for a service call to have the lamp replaced and also in lost time while the truck is stopped for maintenance on the lamps.
It has been suggested, for example, in U.S. Pat. Nos. 4,128,860 and 4,216,525 that a reserve or standby lamp may be provided in each marker light fixture. When a primary lamp fails, a transistor circuit automatically switches from the primary lamp to the standby lamp. Since the standby lamp has not been operated up to this point, the filament will be ductile and will not have been subceptable to vibration and induced failure. These patents also suggest that the standby lamp may be either a separate lamp or a second filament combined with the primary filament in a single dual filament bulb. These patents also suggest that a four diode bridge rectifier may be provided so that the circuit may operate on dual polarity.